1. Field of the Invention
The present invention relates to a method and system for the measurement of the concentrations of a reducing agent and a metal ingredient contained in an electroless plating solution. This invention is also concerned with a method for the automatic adjustment of the concentrations of ingredients in an electroless plating solution and a system for practicing the automatic adjustment method.
2. Description of the Related Art
Various methods have heretofore been proposed for the measurement of the concentration of a metal ingredient in an electroless plating solution, including the absorptiometric measurement making use of the color of metal ions itself as proposed in Publication of Unexamined Japanese Patent Application (KOKAI) No. 63-121668, the use of fluorescent x-ray analysis as disclosed in Publication of Unexamined Japanese Patent Application (KOKAI) No. 60-164239, and the adoption of potentiometric titration as described in Publication of Unexamined Japanese Patent Application (KOKAI) No. 62-14053.
Further, regarding the measurement of the concentration of a reducing agent such as formaldehyde in an electroless plating solution, there is a method in which the concentration of the reducing agent is determined from the quantity of electrocity required for the oxidation of the reducing agent as disclosed in Publication of Unexamined Japanese Patent Application (KOKAI) No. 53-9235.
In electroless plating, both metal ingredient and reducing agent are consumed out of ingredients of a plating solution as the plating proceeds. Further, the velocity of electroless plating, namely, the deposition rate depends heavily on the concentrations of the individual ingredients in the plating solution. To maintain the deposition rate, it is necessary to measure the concentrations of both the metal ingredient and the reducing agent and to replenish their consumed amounts from time to time. Among the above conventional techniques, the method making use of the inherent color of a metal ingredient and the method employing potentiometric titration are however accompanied by the problem that they can be applied to only a limited type of electroless plating solutions to measure the concentrations of their metal ingredients and they hence do not have broad applicability. Turning next to the measuring method for the concentration of a reducing agent, nothing has been taken into consideration for electroless plating solutions which contain a great deal of reducing ions, such as SO.sub.3.sup.2-, S.sub.2 O.sub.3.sup.2-, as a complexing agent in addition to the reducing agent.
There have been known, for example, non-cyanide electroless gold plating solutions which contain gold ions as metal ions, thiosulfate ions (S.sub.2 O.sub.3.sup.2-) and sulfite ions (SO.sub.3.sup.2-) as ligands for forming complexes with the gold ions, and thiourea as a reducing agent. An attempt to determine the concentration of thiourea as the reducing agent from the quantity of electrocity required for its oxidation however does not lead to any accurate measurement of the concentration of the reducing agent because the sulfite ions and thiosulfate ions are oxidized at the same time. Thiourea as a reducing agent can be analyzed by the sodium nitroprusside reaction method, while sulfite ions and thiosulfate ions can be analyzed by ion chromatography. Sodium nitroprusside is however prone to decomposition and the measurement is time-consuming. The sodium nitroprusside reaction method is therefore not suitable for the analysis in actual production although it may be used in laboratory-level analyses.
As has been described above, in the field of electroless plating technology, no technique has been developed yet to permit accurate measurement of the concentrations of ingredients in a plating solution in actual production. This has therefore imposed the problem that control of the concentration of one or more ingredients cannot be performed properly.